The art contains various examples of the construction and operation of inertial measurement units (IMU) for measuring linear acceleration and angular rate. One such IMU is disclosed in U.S. Pat. No. 4,841,773, issued Jun. 27, 1989. The IMU described therein uses a plurality of accelerometer sensor units. Each accelerometer includes a single crystal silicon chip sandwiched between a pair of non-conductive insulative members which may be made from pyrex. The accelerometer includes a proof mass which has been separated from the silicon chip by an etched channel to free the proof mass along at least three of its edges. The fourth edge is provided with a hinge or flexure which may be formed by etching away some, but not all of the edge which joins the proof mass to the silicon chip. Although the patent states that any hinge will do, the embodiment shown in the patent utilizes a cross beam flexure blade hinge.
Sensors used in IMU's must be both sensitive and rugged. For example, in a rocket motor guidance system for a projectile which is fired from a gun, the projectile is accelerated within the gun barrel to an acceleration on the order of 30,000 g's. Immediately after the projectile is expelled from the gun, the projectile undergoes an acceleration in the opposite direction on the same order of magnitude. For these and other applications, it is desirable to provide protection for the sensor device from forces due to such accelerations along the sensitive axis, opposed to the sensitive axis, and in other directions as well. Such protection, however, should not degrade the sensitivity of the sensor device. While attempts have been made to provide stability for sensor devices during normal operation of the device, i.e., with the use of damping plates, none have adequately addressed the problems associated with accelerations that greatly exceed those which the device is designed to withstand. Thus, the prior art sensor designs now offer unsatisfactory performance, particularly upon the receipt of accelerations that greatly exceed the sensitivity range of the sensor.